1. Field of the Invention
The present invention relates to an electrostatic ink-jet head which ejects an ink drop from a pressurizing chamber onto recording paper through a nozzle opening by pressurizing the ink in the pressurizing chamber with an oscillation plate that is electrostatically actuated by an electrode opposite to the oscillation plate. Further, the present invention relates to a method of production of an electrostatic ink-jet head having an electrostatic actuator.
2. Description of the Related Art
Ink-jet heads are provided for ink-jet recording systems, which are included in various image forming systems such as printers, facsimiles, copiers, plotters, and other such image forming devices. In a typical, ink-jet head, an ink drop is ejected from a pressurizing chamber onto recording paper through a nozzle by pressurizing the ink in the pressurizing chamber. The pressurizing chamber communicates with the nozzle. An energy generating device is provided to generate energy used to pressurize the ink in the pressurizing chamber. When a driving voltage is applied to the energy generating device, the energy generating device generates the energy so that the ink-jet head ejects an ink drop from the nozzle onto the recording paper by pressurizing the ink in the pressurizing chamber. Currently, the on-demand ink-jet head is dominant in the related art. In ink-jet heads of this type, the head ejects ink drops onto the recording paper only when recording is required.
For example, as disclosed in Japanese Published Patent Application No. 2-51734, a conventional ink-jet head includes a plurality of nozzle openings arranged parallel to each other so as to eject ink drops therefrom, a plurality of pressurizing chambers respectively attached to the nozzle openings, each pressurizing chamber having walls one of which is arranged to function as a diaphragm, a plurality of piezoelectric elements respectively attached to the corresponding diaphragms, and a common ink cavity for supplying ink to each of the pressurizing chambers.
When a driving voltage is applied to any one of the piezoelectric elements in the above-mentioned ink-jet print head, the diaphragm corresponding to the one of the piezoelectric elements is mechanically deflected so that the volume of the pressurizing chamber corresponding to the diaphragm is instantaneously reduced and the ink in the pressurizing chamber is pressurized. As a result, an ink drop is ejected from the corresponding nozzle opening onto the recording paper.
However, the above piezoelectric type ink-jet print head requires much time and labor for mounting the piezoelectric elements on the pressurizing chambers because the piezoelectric elements must be attached to the outside of the pressurizing chambers through glass or resin plates forming the diaphragms or must be arranged in the inside of the pressurizing chambers. Further, it is very difficult for the piezoelectric ink-jet head to achieve high-speed, high-quality printing because the piezoelectric ink-jet head must be equipped with a larger number of nozzle openings for ejecting ink drops.
Further, as disclosed in Japanese Published Patent Application No.61-59911, another conventional ink-jet head is also known. In this ink-jet head, the ink in the pressurizing chamber is heated by a heater, so that the pressure in the pressurizing chamber is increased by the bubbles generated by evaporation of the ink. As a result, the ink-jet head ejects ink drops from the pressurizing chambers.
However, the above-mentioned heating system has a problem in that the operational life of the head is comparatively short because the heating resistors are damaged by the repetition of heating/cooling and by the shock at the time of the breaking of bubbles in the ink.
In order to eliminate the problems of the above conventional heads, an electrostatic ink-jet head has been proposed. As disclosed in Japanese Laid-Open Patent Application No. 6-71882, the electrostatic ink-jet head includes a plurality of nozzle openings, a plurality of pressurizing chambers respectively attached to the nozzle openings, diaphragms respectively disposed in the corresponding pressurizing chambers, a plurality of driving electrodes for respectively driving the corresponding diaphragms, and a common ink cavity for supplying ink to the plurality of pressurizing chambers. Each of the diaphragms defines a bottom of one of the pressurizing chambers. The diaphragms and the pressurizing chamber walls are arranged parallel to each other, and a parallel gap is provided between each diaphragm and the corresponding pressurizing chamber.
In the above-mentioned ink-jet print head, when driving pulses are applied to the driving electrodes, the driving electrodes respectively actuate the diaphragms via an electrostatic force in a direction to increase the pressures in the respective pressurizing chambers to eject ink drops from the nozzle openings onto recording paper.
In the electrostatic ink-jet head of the above type, the amount of displacement of the diaphragm and the electrostatic attraction pressure on the diaphragm, when the diaphragm is driven, are calculated as follows.
Suppose that the diaphragm has a rectangular shape including a short-side length xe2x80x9c2axe2x80x9d and long-side length xe2x80x9cbxe2x80x9d. The amount of displacement xcex4 (m) of the thin plate (the diaphragm) is represented by
xcex4=kxc2x7{12(1xe2x88x92xcexd2)/Eh2}xc2x7Pa4xe2x80x83xe2x80x83(1)
where xe2x80x9ckxe2x80x9d is a constant, xe2x80x9cxcexdxe2x80x9d is Poisson""s ratio, xe2x80x9cExe2x80x9d is Young""s modulus (N/m2), xe2x80x9chxe2x80x9d is a thickness (m) of the diaphragm, and xe2x80x9cPxe2x80x9d is an electrostatic attraction pressure (N/m2) between the diaphragm and the electrode.
Further, in the electrostatic inkjet head of the above type, the electrostatic attraction pressure P (N/m2) between the diaphragm and the electrode is represented by:
xe2x80x83P=(1/2)xc2x7∈xc2x7(V/t)2xe2x80x83xe2x80x83(2)
where xe2x80x9c∈xe2x80x9d is a dielectric constant (F/m), xe2x80x9cVxe2x80x9d is the applied voltage (V), and xe2x80x9ctxe2x80x9d is a distance (m) between the diaphragm and the electrode.
In order to mount a larger number of nozzle openings on the electrostatic ink-jet head of the above type for achieving high-speed, high-quality printing, it is necessary to reduce the short-side length xe2x80x9caxe2x80x9d of each of the diaphragms. However, as is apparent from the above equation (1), if the length xe2x80x9caxe2x80x9d is reduced, the amount of displacement of the diaphragm is greatly decreased. Hence, the short-side length xe2x80x9caxe2x80x9d of the diaphragm must be maintained at a certain level.
In order to allow adequate amount of displacement of the diaphragm for ejecting a proper amount of ink while maintaining the short-side length xe2x80x9caxe2x80x9d of the diaphragm at a certain level, it is necessary to meet any of the following requirements in accordance with the equations (1) and (2): (i) the diaphragm thickness xe2x80x9chxe2x80x9d must be reduced; (ii) the distance xe2x80x9ctxe2x80x9d between the diaphragm and the electrode must be reduced; and (iii) the driving voltage xe2x80x9cVxe2x80x9d must be increased.
Regarding the requirement (i) above, if the thickness xe2x80x9chxe2x80x9d of the diaphragm is reduced, the rigidity of the diaphragm is greatly decreased, which will significantly lower the ink ejecting pressure of the diaphragm. Regarding the requirement (ii) above, if the distance xe2x80x9ctxe2x80x9d is reduced, the maximum amount of displacement of the diaphragm is decreased, which will considerably reduce the amount of ink drops ejected by the head and will cause defective printing on the recording paper. Regarding the requirement (iii) above, if the driving voltage xe2x80x9cVxe2x80x9d is increased the cost is considerably increased.
Accordingly, it is desired to provide an electrostatic ink-jet head which has a construction that overcomes the problems described above and achieves adequate amount of displacement of the diaphragm for ejecting a proper amount of ink while maintaining the short-side length of the diaphragm at a certain level.
Japanese Laid-Open Patent Application No. 9-39235 discloses an electrostatic ink-jet head in which a pressurizing chamber is provided and a bottom wall of the pressuring chamber partially defines an oscillation plate. Driving electrodes are arranged on step-wise surfaces of a base substrate, which face the oscillation plate on the bottom of the pressurizing chamber. The step-wise surfaces of the base substrate are arranged in a staircase configuration with steps having different heights. Hence, step-wise gaps are provided between the driving electrodes and the oscillation plate, and the step-wise gaps are decreased in the distance between each of the respective electrodes and the oscillation plate in a direction away from the position just below the nozzle opening.
Japanese Laid-Open Patent Application No.9-193375 discloses an electrostatic ink-jet head in which a pressurizing chamber is provided and a bottom wall of the pressuring chamber partially defines an oscillation plate. A driving electrode is provided on a linearly sloped surface of a base substrate, which faces the oscillation plate on the bottom of the pressurizing chamber. The linearly sloped surface of the base substrate, on which the electrode is mounted, is arranged in a non-parallel manner relative to the oscillation plate, and the gap between the electrode and the oscillation plate is linearly decreased in a direction away from the nozzle opening.
The above-mentioned ink-jet print heads have an advantage in that the oscillation plate can be actuated with a low driving voltage applied to the electrode because the displacement of the oscillation plate generated by the electrostatic force can start from a position where the distance (the gap) between the electrode and the oscillation plate is relatively small. However, the above-mentioned heads have a difficulty in providing efficient low-voltage actuation of the oscillation plate as well as a proper amount of ink ejected by the actuation of the oscillation plate.
Japanese Laid-Open Patent Application No.7-214770 discloses an electrostatic ink-jet head in which an electrode is brought into contact with an oscillation plate when a driving voltage applied to the electrode is above a certain voltage. The ink-jet head has an advantage in that the amount of ink ejected by the head becomes stable. However, it is difficult for the above ink-jet head to provide appropriate ink ejection characteristics that achieve high-speed, high-quality printing.
In order to overcome the problems described above, preferred embodiments of the present invention provide an improved ink-jet head that is constructed and arranged to provide a low-voltage actuation of the oscillation plate as-well as a precise amount of ink ejected by the low-voltage actuation of the oscillation plate.
Further, preferred embodiments of the present invention provide a method of manufacturing an ink-jet head to provide a low-voltage actuation of the oscillation plate as well as a precise amount of ink ejected by the low-voltage actuation of the oscillation plate.
According to one preferred embodiment of the present invention, an ink-jet head is provided with an electrostatic actuator which includes an oscillation plate which defines a bottom of a pressurizing chamber of the ink-jet head, the oscillation plate having a generally rectangular shape with longer sides and shorter sides, the shorter sides having a middle point and peripheral ends, an electrode substrate which is bonded to the oscillation plate, the electrode substrate having a recessed portion that defines an internal space between the oscillation plate and the electrode substrate, and a curved electrode which is provided on the recessed portion of the electrode substrate to face the oscillation plate via the internal space, such that upon application of a driving voltage to the electrode, the electrode actuates the oscillation plate by electrostatic force, so as to pressurize ink in the pressurizing chamber, thereby ejecting an ink drop onto recording paper, wherein a dielectric layer is provided on at least one of the electrode and the oscillation plate, and the recessed portion of the electrode substrate has a generally concave cross-section taken along the shorter sides of the oscillation plate, such that a gap between the electrode and the oscillation plate is reduced from an approximate middle point to the ends of the shorter sides.
According to another preferred embodiment of the present invention, an ink-jet head preferably includes a nozzle opening which ejects an ink drop therefrom onto recording paper, a pressurizing chamber which is attached to the nozzle opening and contains ink therein, an oscillation plate which is provided to define a bottom of the pressurizing chamber, and pressurizes the ink in the pressurizing chamber when the oscillation plate is actuated, the oscillation plate have a substantially rectangular shape with longer sides and shorter sides, the longer sides having a middle point, a first end and a second end, an electrode substrate which is bonded to the oscillation plate, the electrode substrate having a recessed portion that defines an internal space between the oscillation plate and the electrode substrate, and an electrode which is arranged on the recessed portion to face the oscillation plate via the internal space, upon application of a driving voltage to the electrode, the electrode actuating the oscillation plate via electrostatic force, so that the ink-jet head ejects an ink drop from the nozzle opening onto the recording paper by pressurizing the ink in the pressurizing chamber, wherein the electrode has a cross-section taken along the longer sides of the oscillation plate, such that a gap between the electrode and the oscillation plate is reduced from the middle point to at least one of the first and second ends of the longer sides, and the cross-section of the electrode includes a bottom flat region surrounding the middle point and at least one slope region extending from the bottom flat region to one of the first and second ends of the longer sides.
According to another preferred embodiment of the present invention, an ink-jet head preferably, includes a nozzle opening which ejects an ink drop therefrom onto recording paper; a pressurizing chamber which is attached to the nozzle opening and contains ink therein, a substantially rectangular oscillation plate which defines a bottom of the pressurizing chamber, and pressurizes the ink in the pressurizing chamber when the oscillation plate is actuated, the oscillation plate having longer sides and shorter sides, the shorter sides having a middle point, a first end and a second end, an electrode substrate which is bonded to the oscillation plate, the electrode substrate having a recessed portion that defines an internal space between the oscillation plate and the electrode substrate, and a curved electrode which is arranged on the recessed portion to face the oscillation plate via the internal space, such that upon application of a driving voltage to the electrode, the electrode actuates the oscillation plate by electrostatic force, so that the ink-jet head ejects an ink drop from the nozzle opening onto the recording paper by pressurizing the ink in the pressurizing chamber, wherein the recessed portion of the electrode substrate has a cross-section taken along the shorter sides of the oscillation plate, such that a rate of reduction of a gap between the electrode and the oscillation plate in a first direction from the middle point to t he first end of the shorter sides differs from a rate of reduction of the gap in a second direction from the middle point to the second end.
According to another preferred embodiment of the present invention, an ink-jet head preferably includes a nozzle opening which ejects an ink drop therefrom onto recording paper, a pressurizing chamber which is attached to the nozzle opening for containing ink therein, a substantially rectangular oscillation plate which defines a bottom of the pressurizing chamber, and pressurizes the ink in the pressurizing chamber when the oscillation plate is actuated, an electrode substrate which is bonded to the oscillation plate, the electrode substrate having a recessed portion that defines an internal space between the oscillation plate and the electrode substrate, and a curved electrode which is provided on the recessed portion to face the oscillation plate via the internal space, such that upon application of a driving voltage to the electrode, the electrode actuates the oscillation plate by electrostatic force, wherein the recessed portion of the electrode substrate has a first cross-section taken along shorter sides of the oscillation plate, such that a gap between the electrode and the oscillation plate is reduced from a middle point to peripheral ends of the shorter sides, and wherein a protective layer is provided on the electrode, and at least one of the protective layer and the electrode is brought into tangential contact with the oscillation plate at the peripheral ends of the shorter sides.
According to another preferred embodiment of the invention, a method of production of an ink-jet head provided with an electrostatic actuator, the electrostatic actuator including an oscillation plate defining a bottom of a pressurizing chamber of the ink-jet head, the oscillation plate having a substantially rectangular shape with longer sides and shorter sides, an electrode substrate bonded to the oscillation plate, the electrode substrate having a recessed portion that defines an internal space between the oscillation plate and the electrode substrate, and a curved electrode arranged on the recessed portion of the electrode substrate to face the oscillation plate via the internal space, includes the steps of forming a photo-resist layer on the electrode substrate, forming a recess in the photo-resist layer through a photolithography process, the recess corresponding to the recessed portion of the electrode substrate, and producing the recessed portion of the electrode substrate through an etching process by using the photo-resist layer with the recess, wherein the recessed portion of the electrode substrate is configured with a generally concave cross-section taken along the shorter sides of the oscillation plate, such that a gap between the electrode and the oscillation plate is reduced from a middle point to peripheral ends of the shorter sides.
According to another preferred embodiment of the invention, a method of production of an ink-jet head, the ink-jet head including a substantially rectangular oscillation plate defining a bottom of a pressurizing chamber, an electrode substrate bonded to the oscillation plate, the electrode substrate having a recessed portion that defines an internal space between the oscillation plate and the electrode substrate, and a curved electrode provided on the recessed portion to face the oscillation plate via the internal space, includes the steps of forming a photoresist layer on the electrode substrate, performing a photolithographic process to form a recess in the photoresist layer by using a photoresist, the photoresist including a light-transmitting portion having different transmittances and producing the recessed portion of the electrode substrate through an etching process by using the photo-resist layer with the recess, wherein the oscillation plate has shorter sides with a middle point, a first end and a second end, and the recessed portion of the electrode substrate is configured with a cross-section taken along the shorter sides of the oscillation plate, such that a rate of reduction of a gap between the electrode and the oscillation plate in a first direction from the middle point to the first end of the shorter sides differs from a rate of reduction of the gap in a second direction from the middle point to the second end of the shorter sides.
In the ink-jet head of preferred embodiments of the present invention and the method of production of the same, the recessed portion of the electrode substrate preferably has a concave cross-section taken along the shorter sides of the oscillation plate, such that the gap between the electrode and the oscillation plate is reduced from the middle point to the ends of the shorter sides. The oscillation plate can be easily and properly actuated with a low driving voltage applied to the electrode because the displacement of the oscillation plate generated by electrostatic force can start from the peripheral positions where the distance (the gap) between the electrode and the oscillation plate is relatively small. The ink-jet head of preferred embodiments of the present invention is efficient in providing a low-voltage actuation of the oscillation plate as well as a proper amount of ink ejected by the low-voltage actuation of the oscillation plate. Further, it is possible for the ink-jet head production method of preferred embodiments of the present invention to easily produce the ink-jet head with low cost by suitably using photolithography and etching processes.